A lithium-ion secondary battery is a secondary battery whose charged and discharged capacities are high, and which makes it feasible to output high power. At present, lithium-ion secondary batteries have been used as the power source for portable electronic appliances. Furthermore, it is expected to be the power source for electric automobiles that have been predicted to become widely used from now on. However, when using them for these applications, in particular, when using them as a power source for automobile, it has been sought for cutting down their costs and making them more likely to save space. Moreover, as for the use for portable electric appliances, their current major application, it has been desired to make them much shorter, smaller, lighter and thinner.
In lithium secondary batteries that have been employed currently, those which use rare resources, such as cobalt and nickel that are called rare metals, as the positive-electrode electrode material, make a mainstream. Consequently, battery materials, which are more advantageous in view of resourcefulness, have been desired.
Sulfur is a material that is abundant and inexpensive in view of resourcefulness. Besides, sulfur is a material that theoretically exhibits the maximum electric capacity among known positive-electrode materials when it is used as a positive-electrode active material for lithium-ion secondary battery. From Sulfur, it is believed that an electric capacity is obtainable, electric capacity which is larger by about six times, compared with those obtainable from lithium cobaltate positive-electrode materials that have been employed mostly among currently commercially-available positive-electrode materials. Consequently, it has been desired to put sulfur into practical use as a positive-electrode material.
However, compounds of sulfur and lithium are soluble in non-aqueous-system solvents, such as ethylene carbonate and dimethyl carbonate, which have been used as the non-aqueous-system electrolytic solution for lithium-ion secondary battery. Consequently, when compounds of sulfur and lithium are used as a positive-electrode material, there is such a problematic issue that the resulting positive electrodes deteriorate gradually and hence the resultant battery capacities decline because the compounds of sulfur and lithium elute into electrolytic solutions. Moreover, in order to inhibit compounds of sulfur and lithium from eluting into electrolytic solutions, reports have been made on using polymer electrolytes or solid electrolytes. However, since batteries, in which polymer electrolytes or solid electrolytes are used, exhibit high internal resistances and are less likely to be activated or operated at room temperature or lower temperatures, it is necessary to activate or operate them at higher temperatures. Moreover, batteries, in which polymer electrolytes or solid electrolytes are used, also associate with such a problem that the outputs are low, and so forth.
Therefore, when a sulfur-containing material can be realized practically as a positive-electrode material for lithium-ion secondary battery by suppressing the elution of sulfur into non-aqueous-system solvents, it is possible to realize increasing the resulting capacity of lithium-ion secondary battery, and making the resultant lithium-ion secondary more lightweight as well as more likely to save space. Moreover, when it is possible to use, not polymer electrolytes or solid electrolytes, but an electrolytic solution comprising a non-aqueous-system solvent, it becomes feasible to activate or operate the resulting lithium-ion secondary battery at room temperature, or even at lower temperatures.
As one of the attempts to suppress the elution of sulfur into non-aqueous-system solvents, a sulfur-system polymeric substance, which is linked one after another by —CS—CS— bonds and —S—S— bonds, has been proposed (see Non-patent Literature No. 1 mentioned below). However, in a case where this sulfur-system polymeric substance is used as a positive-electrode material, the polymer has been cut off because Li and S bond with each other at the time of discharging. Consequently, the reversibility of reaction has lost, and so the cyclability of the resulting battery has declined.
Moreover, in Patent Literature No . 1 mentioned below, there is set forth a carbon polysulfide whose major components are carbon and sulfur. It is allegedly said that this carbon polysulfide is satisfactory in stability and is good in the resulting charge/discharge cyclability. However, as set forth in Example No. 9 in which an aluminum foil was used as the current collector, for instance, it cannot be said that the resultant cyclability was improved sufficiently because the resulting discharged capacity, which showed 610 mAh/g per active material at the 10th cycle of charging and discharging operations, had deteriorated down to 146 mAh/g at the 50th cycle. As causes of this declining in the discharged capacity, it is possible to believe as follows: since the carbon polysulfide has a structure which is made by adding sulfur to straight-chain unsaturated polymers, the —CS—CS— bonds and the —S—S— bonds are cut off easily during the charging/discharging cycles; and hence the polymers have turned into low molecular-weight substances to dissolve in the electrolytic solution.
Moreover, in addition to those mentioned above, investigations for upgrading the cyclability of lithium-ion secondary battery have been recently carried out variously by means of loading sulfur onto supports such as carbon. However, when investigations on the cyclabilities of batteries having these supports were carried out using a binder resin (e.g., polyvinylidene fluoride (or PVDF)) that has been usually used at present, it was understood that the discharged capacities of the resulting batteries have declined.
It was understood that a cause of this declining in the discharged capacities is that the resistances within the resulting electrodes become larger due to the changes in the states of active material (e.g., expansions, and the like), changes which take place in the process of cyclic tests when PVDF is used. Although it has been unclear what causes this increase in the resistances, as one of the possibilities, it is possible to believe as follows: conductive paths being formed of conductive additives are cut off by means of the expansions of active materials; as a result, the resistances increase.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2002-154,815; and
Non-patent Literature No. 1: “Polymer Lithium Battery,” Written by UETANI Yoshio, and Published by CMC Co., Ltd.